This invention relates to semiconductor devices and circuits and methods for driving LEDs in lighting and display applications.
LEDs are increasingly being used to replace lamps and bulbs in lighting applications, including providing white light as a backlight in color liquid crystal displays (LCD) and high definition televisions (HDTV). While LEDs may be used to uniformly light the entire display, performance, contrast, reliability, and power efficiency are improved by employing more than one string of LEDs and to drive each string to a different brightness corresponding to the portion of the display that the particular LED string illuminates. The benefits of controlling LED string brightness are many. In some cases, the brightness of each string of LEDs can be adjusted in proportion to the brightness of the specific portion of the LCD image being illuminated. For example, the LEDs behind the image of the sun may be biased to full brightness, while in the same video frame, images in shadow or underwater may be more dimly illuminated, emphasizing image and color contrast across the picture. In other cases, the screen may be backlit in horizontal bands, where the portion located immediately behind changing pixels is blackened or dimmed to reduce image blurring associated with the slow phase change of the liquid crystal. “Local dimming” therefore refers to backlighting systems capable of such non-uniform backlight brightness. The power savings in such systems can be as high as 50% as compared with LCDs employing uniform backlighting. Using local dimming, LCD, contrast ratios can approach those of plasma TVs.
To control the brightness and uniformity of the light emitted from each string of LEDs, special electronic driver circuitry must be employed to precisely control the LED current and voltage. For example, a string of“m” LEDs connected in series requires a voltage equal to approximately 3.1 to 3.5 (typically 3.3) times “m” to operate consistently. Supplying this requisite voltage to a LED string generally requires a step-up or step-down voltage converter and regulator called a DC-to-DC converter or switch-mode power supply (SMPS). When a number of LED strings are powered from a single SMPS, the output voltage of the power supply must exceed the highest voltage required by any of the strings of LEDs. Since the highest forward voltage required cannot be known a priori, the LED driver IC must be intelligent enough to dynamically adjust the power supply voltage using feedback. LED voltages cannot be known with certainty because LED manufacturing naturally exhibits variability in forward voltage associated with manufacturing reproducibility and quality of the man-made crystalline material used to form the LEDs. Stochastic variability, i.e. random variation, is an unavoidable characteristic in manufacturing following the mathematical principles of statistics and probability. While manufacturers seek to minimize this variability, they cannot prevent it entirely. Even though testing and sorting can be used to intelligently combine LEDs into strings with more consistent voltages, such operations undesirably add cost and limit factory throughput, and are therefore avoided whenever possible.
In addition to providing the proper voltage to the LED strings, the backlight driver ID must precisely control the current conducted in each string to a tolerance of ±2%. Accurate current control is necessary because the brightness of an LED is proportional to the current flowing through it, and any substantial string-to-string current mismatch will be evident as a variation in the brightness of the LCDs. Aside from controlling the current, local dimming requires precise pulse control of LED illumination, both in timing and duration, in order to synchronize the brightness of each backlight region, zone, or tile to the corresponding image in the LCD screen.
The prior art's solutions to the need for local dimming limit display brightness and are costly. Attempts to reduce these costs sacrifice necessary features, functionality, and even safety.
Conventional Integrated LED Driver Design and Operation
LED system 1, shown in FIG. 1, comprises a conventional backlight controller integrated circuit (IC) 2 with “n” channels of integrated drivers 12A through 12n. For clarity, only channel 12A is shown in detail, but channel 12A represents the other channels as well. The number of integrated channels in a driver IC generally may range from eight to sixteen. As shown, channel 12A comprises a controlled current sink device or circuit 17A in series with a corresponding LED string 3A of “m” LEDs, powered by a controlled voltage supply +VLED.
Similarly, channel 12B (not shown) comprises a controlled current sink device or circuit 17B in series with a corresponding LED string 3B of “m” LEDs powered by the same controlled voltage supply +VLED. Generalizing, the nth channel in driver IC2, i.e. channel 12n, comprises a controlled current sink device or circuit 17n in series with a corresponding LED string 3n of “m” LEDs powered by the same controlled voltage supply +VLED powering all n channels. It should be understood that explanations identifying a specific channel, e.g. channel 12A, apply equally to any channel and collectively to all “n” channels.
In color LCD backlighting applications, the LEDs are typically white LEDs. The color of each pixel is achieved by employing a red, green or blue color filter sitting atop the LCD, changing the white light generated by the LCD and passing through the filter into color by removing the unwanted colors in each region. The brightness of each string of LEDs depends on the current flowing through it, provided that there is adequate voltage to power the string. Excess voltage present across any given string of LEDs 3A-3n, will be absorbed by the corresponding current sink device 17A-17n and can lead to overheating in a specific device. Without integrated thermal protection, the excess heat may damage the corresponding current sink device 17A-17n and the entire integrated circuit 2.
Controlling the currents in current sink devices 17A-17n and LED supply voltage +VLED requires a significant amount of associated circuitry. For example, in addition to current sink device 17A, channel 12A also includes a pulse-width-modulation PWM controller 16A, a digital-to-analog (D/A) controller 15A, an LED fault detector comparator 14A, and a current-sense feedback CSFB amplifier 13A. These elements are duplicated in one-to-one correspondence in channels 12B-12n (e.g., channel B contains a PWM controller 16B and channel n contains a PWM controller 16n, etc.). Through a digital SPI bus interface 4, backlight controller IC 2 therefore independently controls the current in “n” channels of LED strings, each channel having “m” LEDs connected in series in a string. Commands arriving at the SPI bus interface 4 usually come from a microcontroller, a custom ASIC, a field programmable gate array (FPGA), a dedicated graphics IC, or a video processor and scalar IC. The SPI bus, an acronym for “serial peripheral interface” bus, is one common communication standard used in video systems.
The number of series-connected LEDs “m” in each string may vary from 2 to 60, depending on the size, performance, and cost of the TV or LCD, but 10 to 20 is common. The number of channels per backlight controller IC varies by design, but each backlight controller IC typically contains no fewer than 8 channels to limit the number of backlight controller ICs, and no more than 16 channels to avoid overheating, especially at higher currents.
While current sink device 17A generally comprises a high-voltage MOSFET biased as a current mirror, precise current control likely requires active feedback to minimize the influence of drain-to-source voltage on current regulation. In FIG. 1, this feedback circuit is depicted schematically as feedback loop 19A, but in reality, the feedback circuit is generally implemented with amplifiers and additional active and passive devices. The current sink devices 17A through 17n in channels 2A through 2n, respectively, are designed with identical circuit components and ideally similar device orientations to minimize any process-induced mismatch, and in addition the current sink devices may be actively trimmed to improve absolute accuracy and channel-to-channel matching to a tolerance of less than ±2%.
Although the current in any one channel may be varied through the digital SPI-bus interface 4, the maximum current of every channel is set “globally” by the value of an external precision resistor 21 connected to a bias circuit 22. The maximum per channel current, which may range by application and display size from 30 mA to over 300 mA, is therefore a global variable affecting all “n” channels equally within a given backlight controller IC. If two or more backlight controller IC's are used in a system (e.g., a TV), precision resistors must be used to insure acceptable chip-to-chip current matching among all the channels in the system.
The maximum voltage of the high-voltage power device represented by current sink device 17A is depicted schematically by a P-N diode 18A, and may vary by application and display size from 30V to as high as 300V. Typical voltages range from 40V to 100V, where 40V is sufficient to operate ten series-connected LEDs and 100V is suitable for 25 series-connected LEDs. While any single channel can be designed to operate at both the highest voltage and the highest current, the total power dissipation in IC 2 may limit the actual combination of currents, voltages, and number-of-channels practically realizable to avoid overheating and reliability problems. This fundamental thermal limit and the unavoidable tradeoff between the number of channels integrated in the IC and the maximum power delivered by any single channel will be elaborated on later in this disclosure.
To control the duration and timing of illumination of LED string 3A, current sink device 17A is pulsed on and off using pulse-width modulation controlled by PWM controller 16A in response to a digital value representing a duty factor (D) stored in PWM register 9, a digital phase delay value (Φ) stored in the phase delay register 10, and synchronized to the grey scale clock input GSC and the vertical sync signal input Vsync. PWM controller 16A comprises a counter clocked by the grey scale clock signal GSC to generate on-off pulses controlling the current sink device 17A thereby enabling dynamic adjustable LED brightness control.
At the leading edge of the Vsync signal, the digital values of the duty factor (D) and phase delay (Φ) are loaded into the counter within PWM controller 16A, and the counting of the GSC pulses commences. Both the PWM and phase delay digital words are typically 12 bits in length, providing for 4096 different values of phase delay and 4096 different levels of PWM brightness. Phase delay is used to prevent current spikes resulting from simultaneous LED turn-on and to compensate for propagation delay across the display panel. At the onset of counting, the counter within PWM controller 16A counts the phase delay value Φ, during which time, the output of PWM controller 16A remains low, the current sink device 17A remains off, and the LEDs in string 3A remain dark. After the phase delay count 4 loaded from phase delay register 10 is complete, the output of PWM controller 16A goes high, the current sink device 17A turns on, and the LED string 3A becomes illuminated for a duration represented by the duty factor value D loaded from PWM register 9.
The entire sequence described above occurs within one Vsync period, generally repeating at a frame rate of 60, 120, 240, 480 or 960 Hz depending on the display design. During this interval, new values of data for the next picture frame are sent to IC 2 through SPI bus interface 4 and loaded into PWM register 9 and phase delay register 10, respectively. Generally, the grey-scale-clock signal GSC is generated from the Vsync signal by the system controller. Alternatively, a phase lock loop circuit may be employed within IC 2 to internally generate the GSC signal.
Because the GSC signal is synchronized to the Vsync signal, multiple driver ICs may be used in tandem to illuminate larger displays without encountering synchronization issues. Timing information of the GSC and Vsync signals is input into IC 2 through a buffer and timing circuit 11 before being distributed throughout the integrated circuit. An enable pin En is also included as a hardware “chip-select” function, redundant to SPI bus control but useful in start-up sequencing, failure analysis and debugging, and during engineering prototype development.
Unlike a simple MOSFET switch, current sink device 17A represents a high voltage MOSFET biased as a current sink conducting a fixed and calibrated current when it is on and carrying significantly less than a microampere of current when it is off. The actual current during conduction is set globally for all channels by resistor 21 and bias circuit 22, and for the specific channel 12A by the “Dot ILED” digital word stored in a Dot register 8. The term “dot correction” historically relates to adjusting, i.e. calibrating, pixel “dots” to produce uniform brightness to compensate for irregularities and non-uniformity in a display. Today, the current in backlighting applications is generally adjusted for overall display brightness but not to correct for pixel variation across a display, primarily because driving white LEDs at differing currents can change the color temperature, i.e. the spectrum of emitted light, of the white LED strings.
Since the gate voltage and the resulting saturation current in a MOSFET biased as a current sink are analog parameters, a D/A converter 15A is required to convert the digital “Dot” word into an analog voltage to properly drive the MOSFET operating as current sink device 17A. A feedback circuit 19A must be calibrated in conjunction with D/A converter 15A to produce the proper current at full and intermediate brightness codes. An 8-bit word for the Dot parameter is typical, but in some cases 12 bits of resolution are necessary. In monolithic implementations of IC 2, the high-voltage MOSFET implementing current sink device 17A may be divided into sections with 8 to 12 separate gates, digitally weighted to produce 256 to 4096 distinct levels of current. As such, the MOSFET in current sink device 17A performs part of the D/A function, merging D/A converter 15A, in part, into current sink device 17A. Obviously, this implementation would not be practical in multi-chip implementations of LED backlighting units.
In LED backlighting applications, the drain voltage of the MOSFET within current sink device 17A is monitored both to detect LED fault conditions such as open or shorted LEDs, and to facilitate feedback to the voltage regulator supplying the high voltage supply voltage +VLED. Specifically, the analog fault detector comparator 14A monitors the current in current sink device 17A and compares it to a value set by an LED fault register 7. If the voltage rises above a programmed value, e.g. above 6V, then the state of fault detector comparator 14A changes to indicate that a fault condition has occurred, and the change is latched into LED fault register 7. An open drain MOSFET used to generate an interrupt signal is also turned on, pulling the “fault” signal line low to inform the system microcontroller that a fault has occurred. The system must then query fault register 7 for all the ICs in the system to determine which channel has experienced the fault condition.
Detecting a string with a shorted LED is an important requirement for display safety, since a string with a shorted LED will subject the remaining (m−1) LEDs in the string to excessive voltage, a voltage which must necessarily be absorbed by all the other current sink devices 17A through 17n, risking overheating of IC 2. Some manufacturers prefer to disable any string with a shorted LED, fearing that the reason for the short may degenerate into a potentially catastrophic failure in the LED, the LED string or in the printed circuit board, possibly leading to fire.
An over-temperature sensor register 6 can only detect overheating of the entire IC 2; it cannot sense overheating in a specific channel. Shorted LED detection is therefore preferable to temperature sensing, since it can identify a string with a shorted LED at risk of overheating and can proactively shut off that string long before IC 2 overheats. LED fault register 7, along with temperature sensor register 6, both report fault conditions to the system through SPI bus interface 4. Like the shorted LED detect function, over temperature sensing in over-temperature sensor register 6 also includes an open drain MOSFET used to generate an interrupt signal, pulling the “fault” signal line low to inform the system microcontroller that a fault has occurred. Shorted LED detection and over-temperature sensing thereby share the same fault pin. Only through the SPI interface can the system controller ascertain the nature of a fault condition.
The voltage across current sink device 17A is also used to generate a feedback signal needed to power the LED high voltage power supply +VLED. An amplifier 13A represents this voltage monitor, sensing the voltage needed to properly bias the current sink device 17A with sufficient voltage to maintain a constant current, i.e. to avoid the “drop-out” condition where there is no longer enough voltage to meet the current requested by Dot register 8. The current feedback signal represented by diode 18A is therefore also used in determining this minimum voltage for channel 12A, hence the moniker “current sense feedback” and its associated acronym CSFB. Each channel duplicates this sensing and amplifier circuitry. A CSFB circuit 5 compares the voltage of all “n” channels in IC 2 against its input CSFBI and outputs an analog voltage CSFBO equal to the “lowest” of all the internal voltages. The lowest current sink voltage equals the LED string with the highest series LED forward voltage. In this manner, the highest LED string voltage driven by IC 2 is fed back to the system's LED power supply +VLED.
In addition to the foregoing digital, analog and high voltage circuitry, IC 2 includes a high-voltage linear regulator and bias circuit 22 to step down the input voltage VIN, typically 12V or 24V, to the voltages required inside IC 2. One such voltage Vcc, typically 5V, is used as an intermediate supply voltage for most of the control circuitry and therefore requires external filter capacitor 20. The same bias circuitry may also include the constant current reference supply Iref used in current mirrors and for globally setting the maximum channel current for all “n” channel outputs. A precise constant reference current is achieved by biasing external precision resistor 21 with a constant voltage derived from the regulated supply voltage Vcc.
SPI bus interface 4 is a high-speed, albeit complex, bus used to facilitate communication between the system microcontroller and one or more driver ICs. The interface requires 4-pins per driver IC, comprising two data lines, a dedicated clock line and a chip select line. In backlighting, a 4-state 2-pin chip address is commonly used to uniquely identify up to 16 different drivers. Thus up to 16 driver ICs can share one common 4-wire data bus interface, avoiding the need for customized manufacturing of the IC for each address.
Together with the chip address lines, the implementation of SPI bus interface 4 requires 6-pins per IC. This pin count precludes the use of the SPI bus interface in low-cost, low-pin-count packages. For example in a 16-pin package, a 6-pin SPI bus interface will consume 40% of the available pins. Including power and ground, in an 8-pin package, a SPI bus interface leaves no pins for any circuitry or loads.
In driver IC 2, power, bias, timing, enable, and CSFB, and 4 pins for ground (separated into analog ground, power ground and digital ground), together require 11 pins. Adding 6-pins for SPI bus interface 4, and 4 pins for fault settings and fault monitoring, the minimum number of pins for driver IC 2 is 21, plus the number of output channels. An eight-channel driver would therefore require a minimum of a 27-pin package while a sixteen-channel driver requires a package with at least 35 pins. Unfortunately, high-pin count packages, such as 32 and 40 pin packages, are not cheap. Their high-cost adversely impacts the potential gross margin for manufacturers of LED backlight driver ICs and ultimately limits the future cost reductions possible using this conventional architecture.
Repartitioning the functions of the IC shown in FIG. 1 differently in an attempt to reduce packaging cost is problematic in this present day system and IC architecture. Specifically, system 1 and driver IC 2 represent a highly interconnected design, with a large number of analog signals and digital busses distributed throughout the chip. For example, a 12-bit bus may connect SPI bus interface 4 to PWM register 9, another 12-bit bus may connect SPI bus interface 4 to phase delay register 10, an 8-bit bus may connect SPI bus interface 4 to Dot register 8, and a number of other bits may be needed for fault sensing and reporting. Because of the large number of interconnecting busses, SPI bus interface 4 cannot easily be separated from its associated registers 6 through 10.
Similarly, registers 6 through 10 cannot easily be separated from drive and sense circuitry 13A through 16A that drives and controls current sink device 17A. PWM controller 16A is connected to PWM register 9 and phase delay register 10 by two 12 bit parallel busses, D/A converter 15A requires at least a 8-bit wide bus interconnect to Dot register 8. Together, these on-chip busses comprise more than 32 interconnects just to drive channel 12A. If IC 2 contains 16 channels, hundreds of interconnects are necessary. Registers 6 through 10 cannot therefore be easily physically separated from the drive and sense circuitry 13A through 16A.
Seemingly the only way to repartition the system, eliminate high pin count packages, and reduce heat is to separate current sinks 17A to 17n from their associated drive circuitry. While this approach may initially seem attractive, it actually makes matters worse. Specifically, a minimum of 3 connections per current sink is required, one for sensing the current, a second to drive the device, and a third to sense the voltage across the device. So removing current sink from driver IC 2 increases the number of pins on the package for the output channels from 16 pins to 48 pins, tripling the number of pins per channel. In conclusion, the prior art backlighting architecture has no means to eliminate high-pin-count packages.
While eliminating the high cost of high-pin-count packages represents an important and much-needed goal, the cost of the LEDs themselves, not the cost of packaging, is the most significant cost factor in today's state-of-the art LED backlighting systems.
FIG. 2 illustrates a LED backlight system 50 comprising a graphics processor or video scalar IC 54, the source of the video signal in a display or TV, an FPGA or microcontroller (μC) 53, a switch-mode power supply (SMPS) 75, sixteen driver ICs 51A through 51P (collectively referred to as driver ICs 51), each of driver ICs 51 driving sixteen LED strings 57A-57P through 72A-72P. Specifically, driver IC 51A drives LED strings 57A through 57P, driver IC 51B drives LED strings 58A through 58P, etc. As such backlight system 50 represents a 256 string LED drive solution.
As described previously, driver ICs 51 are controlled by a common SPI bus 52 generated by μC 53 in response to video information generated by graphics processor or video scalar IC 54. The microcontroller 53 also generates the Vsync and GSC timing signals. If desired, the PWM brightness data and phase delay may be dynamically adjusted for every channel and LED string uniquely for each and every video frame, so long as the data is written to the driver IC before the next Vsync signal pulse arrives. As such, backlighting system 50 facilitates local dimming capability, reduces power consumption, and enhances image contrast, significantly outperforming uniformly illuminated backlit displays.
Conceptually, system 50 may also dynamically adjust the current in each of the LEDs, but in practice these currents are not changed frequently except during mode changes, e.g. switching between 2D and 3D modes in a HDTV. Specifically, in 3D mode, the LED currents are doubled, the Vsync frequency is doubled, and the PWM pulse duration is halved when compared to normal 2D display mode. The doubling of the frequency is needed to alternatively display the left and right eye information without introducing image flicker. Aside from switching between 2D and 3D modes, the LED currents are not normally adjusted except during calibration at the factory during manufacturing.
As shown in FIG. 2, SMPS 75 generates at least two outputs, a regulated 24V supply 74 used to power driver ICs 51A through 51P, and the high-voltage +VLED supply 73, dynamically varied in response to a current sense feedback (CSFB) signal on line 76A. CSFB line 76A carries the CSFB signal that is generated from CSFB circuitry like that shown in CSFB circuit 5 in system 1. The CSFB signal on line 76A is connected in daisy chain fashion with the CSFB signal on line 76B input to driver IC 51A from driver IC 51B, which in turn is connected with the CSFB signal on line 76C from the prior driver IC, and so on. Each of driver ICs 51P outputs CSFB signal representing the lowest current sink voltage of its outputs and of the outputs of all the prior drivers in the daisy chain. Each of lines 76A-76P therefore operates at a different voltage, diminishing in value stage by stage as the CSFB signal approaches SMPS 75. As shown, there is no common line summing or analog “OR”ing the feedback signal from the various driver ICs. The final CSFB signal on line 76A therefore represents the lowest current sink voltage and likewise corresponds to the highest LED string voltage in the entire system. The CSFB signal on line 76A that is input into SMPS 75 may be a voltage or a control current. If a feedback current, rather than a voltage, is required, the CSFB voltage signal can be converted into a current by inserting a transconductance amplifier in the feedback signal path 76A. This is illustrated in FIG. 2 by transconductance amplifier 77 shown in dashed lines.
To summarize, backlight system 50 represents a 256 string LED drive solution.
Assuming that there are four series-connected LEDs per string, the total solution embodied by system 50 utilizes 1,024 LEDs. The cost of this would be too high except for the most expensive high-end HDTVs. Assuming that, with adequate thermal design margins, the maximum current for a 16-channel drive IC is 50 mA per channel, such a system would have a total drive current of 51 LED-amps. (The unit “LED-amps” is the product of the total number of LEDs and the current flowing through each of them, respectively. Since the brightness of an LED is proportional to its current, “LED-amps” is a measure of the luminance, i.e. the total brightness, of a backlight system.)
The foregoing discussion indicates that the only way to reduce the cost of the LEDs and still maintain LED backlight brightness at today's standards is to drive fewer LEDs at higher currents. Higher currents, as it will be shown, increase heating within the driver IC. Furthermore, the only way to eliminate high driver IC costs for a given number of LEDs and still maintain the LED-amps is to use fewer driver ICs. This means that more LEDs must be connected in series and that they must operate at higher voltages. As it will be shown, however, connecting more LEDs in series also increases heating in the driver IC.
In short, the desire to use fewer LEDs and fewer driver ICs to lower costs by operating the LED strings at higher currents and at higher voltages is adverse to achieving safe and reliable LED backlighting solutions immune from overheating.
Thermal Management of Integrated LED Drivers
The major cause of heating in LED driver ICs is not in the intrinsic operation of the IC, but due to mismatch in the forward voltage of the LED strings being driven.
Consider the series-parallel network of LEDs 100 shown in FIG. 3A. A current sink 118 conducting current ILED1 and biased at a voltage Vsink1 drives a string of “m” series connected LEDs 101A through 101m having a total series voltage of Vf1. Similarly, current sink 119 conducting current ILED2 and biased at a voltage Vsink2 drives a string of “m” series connected LEDs 102A through 102m having a total series voltage of Vf2. Likewise an “nth” channel with current sink 133 conducting current ILEDn and biased at a voltage Vsinkn drives a string of “m” series connected LEDs 117A through 117m having a total series voltage of Vfn. All “n” strings are powered by a common shared high voltage supply +VLED at voltage slightly higher than the highest voltage LED string in the system
The voltage Vsink across any given current sink device is then given byVsink=+VLED−Vf 
Unavoidably, the forward voltage of every string of LEDs will vary and therefore randomly mismatch the other strings of LEDs. This mismatch is a natural consequence of the stochastic variation in LED voltage arising from the LED manufacturing process. Without sorting or filtering the natural distribution, we can make a simplifying assumption that the population of any one LED will follow a Gaussian distribution characterized by a mean and standard deviation. We can approximate the mean forward voltage of a string of “m” series-connected LEDs by the average voltage Vfave and its variability by the approximationV3σm=V3σ1SQRT(m)
where V3σ1 is the 3-sigma standard deviation of the forward voltage across a single LED and V3σm is the 3-sigma standard deviation of the forward voltage across a string of “m” randomly selected series-connected LEDs. This relationship is shown in FIG. 3B where Vσ1 is assumed to be 0.6V.
Even in the absence of any channel-to-channel mismatch, there is some minimum voltage Vmin ever-present across all the current sink devices needed to maintain their operation as controlled constant-current devices. This minimum voltage, similar to the “drop-out” voltage on a linear voltage regulator, is the minimum drain-to-source voltage drop present across the MOSFET and its associated current sensing element within a current sink device below which it can no longer insure that a constant and controlled current will flow in the LED string it drives. With constant improvement, the minimum voltage across a current sink device is now approximately 0.5V.
Even in the absence of any channel-to-channel mismatch, a minimum drop of a Vmin means every current sink device must dissipate at least Psink(min)≧Vmin·ILED, and an n-channel driver IC will dissipate “n” times that amount. For example, a 100 mA current through the current sink device will dissipate (100 mA)·(0.5V) or 50 mW per channel and a sixteen channel LED driver will therefore necessarily dissipate a total power Ptotal of at least 800 mW with no mismatch in the forward voltage Vf across the respective LED strings.
The actual voltage drop across any given current sink device, however, is normally higher than Vmin. Referring again to FIG. 3A, if we assume that “n” channels of “n” strings of LEDs have an average forward voltage drop Vfave, and that the in a given channel the power supply is biased at a three-sigma voltage above that average forward drop, plus the minimum voltage drop across the current sink device, i.e. where +VLED=V3σm+Vfave+Vmin, then in that channel the above equation becomesVsink=+VLED−Vf=(V3σm+Vfave+Vmin)−(Vfave)=V3σm+Vmin 
Then the power dissipation in an average current sink device isPsink=ILED·(V3σm+Vmin)
which means the voltage due to string-to-string mismatch is additive atop the minimum voltage needed to operate the current sink device above dropout. By combining these two equations to calculate the power dissipated in any average current sink device, we seePsink=ILED·(V3σ1SQRT(m)+Vmin)
The power dissipation in an “n” channel driver IC is then on averagePtotal=n·[ILED·(V3σ1SQRT(m)+Vmin)]
where “n” is the number of integrated channels, “m” is the number of series-connected LEDs in each channel, ILED is the LED current, and V3σ1 is the 3-sigma value for a single LED forward voltage.
This relationship reveals that a driver IC can dissipate too much power Ptotal as a result of the current ILED, the number of channels “n”, or the number of series-connected LEDs “m” in each channel. Because power dissipation involves three independent design variables, it is difficult to envision or represent this relation graphically. Fortunately, rearranging the equation intoPtotal=[n·ILED]·[(V3σ1SQRT(m)+Vmin)]
provides insight, revealing that n·ILED, is simply the total current Itotal being supplied by any given driver IC, i.e. with n-channels each conducting the current ILED. So givenItotal=n·ILED 
then the equation simplifies toPtotal=[Itotal]·[(V3σ1SQRT(m)+Vmcn)]
Thus, for a given system the total power dissipation in a driver IC is the same whether the system includes one LED string conducting 200 mA, two IED strings conducting 100 mA each, or four strings conducting 50 mA each. The total power dissipated in the driver IC is solely a function of the sum total of the currents conducted through the IED strings.
This relationship is illustrated in FIG. 3C where the columns represent the total driver current Itotal for a driver IC ranging from 200 mA to 1 A and the rows represent the number of series LEDs “in”. Each square illustrates the statistically average power dissipation for a driver IC with that design combination.
For example, an LED driver driving two strings of eleven series-connected LEDs (i.e. m=11) with each of the two strings conducting 200 mA (i.e. where n=2, and Itotal=2×200 mA=400 mA), statistically will dissipate an average power of 1 W per driver IC. In general, the higher the number of series connected LEDs “m” and the larger the total driving current [n·ILED], the higher the power dissipation. As such the lower right hand corner represents the hottest, highest power condition, while designs in the upper left hand corner represent the coolest, lowest power designs.
Region 159 in FIG. 3C illustrates operating conditions dissipating power less than 1 W, a level easily manageable by printed circuit board (PCB) design to avoid overheating. For example, a two channel driver carrying 150 mA per string (or 300 mA total) can drive strings of 20 LEDs connected in series without overheating. The current can safely be increased to 200 mA per string (or 400 mA in total) if the number of series LEDs is no more than eleven, i.e. m≦11.
At higher power levels, shown by regions 156 and 157, the package and printed circuit board design significantly affects the die temperature, the maximum power dissipation, and the current handling capability of a driver IC. Region 158 represents poor electro-thermal design choices, leading to spurious or constant overheating problems, long term and short term reliability risks, and even fire hazard.
Region 156 illustrates operating conditions requiring a package and PCB design capable of dissipating 1.5 W. An example of such a design is a 60 mA per channel driver powering eight strings often series connected LEDs, i.e. n=8, m=10, ILED=60 mA. Delivering a total current of 480 mA, the total power dissipation of such an IC is approximately 1.2 W. While many packages are capable of handling that power, care must be taken to insure that the printed circuit board can carry away that amount of heat to maintain safe, reliable operation. This concern is especially important on single-layer PCB designs, since the circuit board has little thermal mass and no efficient way to perform heat transport away from the driver IC.
Region 157 illustrates operating conditions requiring a package and PCB design capable of dissipating at least 2 W. Such designs require a soldered exposed die pad to conduct heat from the driver IC into the printed circuit board copper traces, and likely require a 4-layer PCB. Multi-layer PCBs, because of their sandwich of copper conductive traces, electrical vias, and solid copper ground planes, intrinsically carry and redistribute heat effectively compared to thinner lower cost PCBs. In more expensive “high-end” HDTVs for example, the demand for a high resolution backlight system demands a greater number of lower current LED strings to enhance image contrast. A 5s16p driver design, i.e. where the number of series connected LEDs m=5, and where the number on integrated channels n=16, can deliver 60 mA or 960 mA of total current to the sixteen LED strings and dissipate 1.84 W, still below the 2 W limit shown. In high-end products multi-layer PCBs represent a small and affordable portion of the total display cost. In many other cases, however, such boards are overpriced for the commodity markets they are meant to serve.
The information in FIG. 3C is displayed parametrically in a semilog graph in FIG. 3D with the total driver IC power dissipation on the y-axis plotted against the number of series connected LEDs “m” on the x-axis, varied parametrically by the total driver current Itotal shown by curves 161 through 167 at currents of 200 mA, 250 mA, 300 mA, 400 mA, 500 mA, 600 mA, 800 mA and 1000 mA, respectively. The 1 W, 1.5 W and 2 W limits are marked as lines 168, 169 and 170 to delineate the borders of regions 159, 156, 157, and 158 of table 155.
FIG. 3D clearly illustrates that the number of series connected LEDs “m” must be reduced as the current handling capability of the driver IC is increased. At 1.5 W, for example, 600 mA of drive capability limits the maximum number of series connected LEDs to around 11, while at 800 mA, the maximum number of series LEDs is half that amount, i.e. m≦5.
FIG. 3D also illustrates that the package power handling demand rises quickly with increasing current. For a design with 10 series-connected LEDs (i.e., m=10), a 1 W package is limited to 400 mA or total drive current, a 1.5 W package is limited to 600 mA, and a 2 W package and PCB design can only safely deliver 800 mA. In an 8-channel driver at these power levels, the total per channel current is therefore thermally limited to 50 mA, 75 mA and 100 mA respectively, currents too low to facilitate lower LED count designs—designs where fewer LED strings are driven at higher currents.
Clearly, the current handling capability of multi-channel LED driver ICs is limited. An alternative approach is to use discrete MOSFETs to implement the current sink, and to drive the discrete MOSFETs by an LED controller IC lacking integrated high voltage drivers. This approach, too, is extremely problematic, as described next.
Driving Discrete Power DMOSFETs as Current Sinks
FIG. 4 illustrates a multichip system 200 for driving the LEDs. The controller architecture is similar to that contained in driver IC 2, except that the multi-channel current sink devices, current sensing elements, and voltage protection devices have been removed from a controller IC 202. Controller IC 202 drives multiple discrete transistor components as current sink devices 217A-217n, using multiple discrete passive components 228A-228n to accurately measure current in the current sink devices 217A-217n and in LED strings 203A-203n. Additional discrete transistor components 225A-225n are optionally employed to clamp the maximum voltage present across the current sink devices 217A-217n, especially for operation at higher voltages, e.g. over 100V. For simplicity's sake, only a single-channel set of components comprising discrete current sink device 217A and transistor component 225A, passive component 228A, together driving LED string 203A, are shown. Each of these “components” is a discrete device in a separate package, requiring its only pick-place operation to position and mount it on its printed circuit board. Each set of three discrete components, along with the corresponding string of LEDs, is repeated “n” times for an “n” channel driver solution.
The active current sink device 217A controlled by IC controller 202 comprises a discrete power MOSFET, specifically a vertical DMOSFET 223A with an intrinsic drain to body diode 224A. Vertical DMOSFET 223A cannot be operated near the avalanche voltage of diode 224A or else hot-carrier damage may result, especially during constant current operation. Typical rated breakdown voltages may vary from 30V to 60V. The gate of the DMOSFET 223A embodying current sink device 217A is driven by the DRIVE output of controller IC 202, specifically the output of an amplifier 216A.
Current measurement and feedback in system 200 utilizes discrete passive component 228A, in this case a precision sense resistor 229A. The voltage on sense resistor 229A provides feedback to the ISENSE pin of controller IC 202. The voltage at the ISENSE pin is buffered by an amplifier 219A and ultimately fed into a gate buffer amplifier 216A. This voltage, proportional to the current flowing in current sink DMOSFET 223A, is compared against the output of a D/A converter 215A in amplifier 216A, the output of which is used to set the current flowing in current sink DMOSFET 223A based on the value of Dot register 208 and the reference current Iref established by bias circuit 222 and set resistor 221. Bias supply 222 regulates input voltage VIN, e.g. 24V, to a lower voltage Vcc, e.g. 5V. This voltage is then used to power the remaining circuit blocks within IC 202. Combined with external set resistor 221, bias circuit 222 establishes internal reference current Iref used to bias D/A converter 215A and ultimately set the maximum current in DMOSFET 223A. The precision in channel-to-channel current matching is set by sense resistor 229A, and by the voltage offset in amplifiers 219A and 216A. Since there are more sources of error in this multichip approach, trimming and the precision of sense resistor 229A are more stringent than circuits where trimming can be performed in closed loop operation.
As in monolithic system 1, SPI bus interface 204 passes PWM brightness and phase delay signals through registers 209 and 210, respectively, the respective outputs of which are subsequently processed by timing and control unit 211 to pulse the output of amplifier 216A, driving the gate of DMOSFET 223A synchronously with the Vsync and GSC signals.
Above 100V operation, discrete transistor component 225A, embodied by a vertical power DMOSFET 226A with high-voltage drain to body diode 227A, is typically added to protect the current sink DMOSFET 223A from damage. The gate of DMOSFET 226A is biased to a fixed voltage, e.g. 12V, and its source is connected in a source-follower configuration to the drain of current sink DMOSFET 223A and its drain is connected to LED string 203A. As a source-follower, the maximum voltage on the source of DMOSFET 226A is limited to a threshold voltage below its gate bias, i.e. to around 10V. Because source-follower operation limits the maximum voltage on the its source, DMOSFET 226A can be viewed as a “cascode clamp”. In this way a lower voltage rating device, e.g. 20V, can be used to realize current sink DMOSFET 223A at a lower cost. Also, since a source-follower operates in its linear region, behaving like a resistor, DMOSFET 226A dissipates much less power than current sink DMOSFET 223A.
The source voltage of “cascode clamp” DMOSFET 226A is also used as the VSENSE input to controller IC 202, feeding the respective inputs of a CSFB amplifier 213A and an LED fault detection comparator 220A. The respective outputs of CSFB amplifier 213A and LED fault detection comparator 220A are in turn connected to a CSFB circuit 205 and an LED fault register 207.
One significant difference between the multichip system 200 and the monolithic driver 1, is that temperature sense circuit 206 can only detect the temperature of IC 202, where no power is dissipated. Unfortunately, the significant heat is generated in discrete current sink DMOSFET 223A, where no temperature sensing is provided. Similarly, the other discrete current sink DMOSFETs 223B-223n likewise have no temperature sensing, and these DMOSFETs could overheat without the system being able to detect or remedy the condition.
In multi-chip system 200, reliable operation of discrete current sink DMOSFET 223A depends on its interconnection with resistor 229A and cascode clamp MOSFET 226A. Each channel of LED drive therefore requires three discrete components—transistor component 225A, current sink device 217A and discrete passive component 228A,—and three connections between these components and controller IC 202.
To illustrate, FIG. 5A shows a simplified, functional view of the multi-chip system 200. each channel of the LED drive requires a VSENSE, DRIVE and ISENSE line on controller IC 202, plus three discrete components 225A, 217A and 228A comprising cascode clamp DMOSFET 226A, current sink DMOSFET 223A and precision resistor 229A.
FIG. 5B illustrates a multi-chip system 270 that is similar to system 200 but in which an Iprecise circuit 282A has been added to beneficially eliminate the sense resistor 229A and the current mismatch and inaccuracy inherent in amplifiers 216A and 219A. Even the simplified system 270 does not eliminate the need for two discrete device components 225A and 217A per channel and does not reduce the number of pins on IC 271 needed to drive and sense the current and voltage in discrete DMOSFETs 226A and 223A.
So in the case using sense resistors, exemplified by multi-chip system 200, one 16-channel controller IC requires 48 discrete components and 48 pins to drive 16 strings of LEDs. Even in the simplified case using an integrated Iprecise feedback circuit, exemplified by multi-chip system 270, a single 16 channel IC requires 32 discrete components and still requires 48 pins plus 3 ground pins, i.e. 51 pins just to drive 16 strings of LEDs.
FIG. 6A illustrates a top view of an expensive, high-pin-count package 301, containing a die 303, of the kind that is typically needed to support controller IC 202. As shown, package 301 is a 72-pin QFN package comprising 51 output pins and 21 interface and control pins. Such a package, 9 mm×9 mm in area, requires a substantial amount of plastic mold compound, copper and many gold bond wires, and as such is intrinsically expensive. In some cases, LCD manufacturers use single-layer printed circuit board manufacturing technology, in which case the 0.5 mm pin pitch and leadless construction of the QFN package is too advanced for their board assembly capabilities. If so, the customer may demand a leaded package with a minimum pin pitch of 0.8 mm, such as a leaded quad flat package (LQFP). To accommodate 72 pins at a 0.8 mm pin pitch, the package size swells to 14 mm×14 mm and the cost increases accordingly.
Aside from the high package expense, the enormous build of material (BOM) component count of a multi-chip LED driver system 350 is shown schematically in FIG. 6B. Driver system 350 requires an expensive high-pin-count controller IC 356, 16 discrete current-sink DMOSFETs 354, 16 discrete cascode clamp DMOSFETs 353, a microcontroller 357 and an SMPS module 351. Collectively, current sink DMOSFETs 354 comprise discrete devices 354A through 354Q, each packaged in a low thermal resistance package having a heat tab, such as an SOT223 package. No temperature sensing is available in the discrete current sink devices 354A through 354Q.
Collectively, cascode clamp DMOSFETs 353 comprise discrete devices 353A through 353Q, each packaged in a conventional leaded surface-mount package, such as an SOT23 package.
As shown, each LED string 352A through 352Q is connected in series with a corresponding cascode clamp discrete DMOSFET 353A through 353Q and a discrete current sink DMOSFET 354A through 354Q, respectively. LED controller IC 356 connects to the current sink devices 354 through 48 conductive traces 359, connecting to each source, gate, and drain with electrically separate and distinct conductive traces. In the embodiment shown in FIG. 6B, LED controller 356 utilizes the internal current sensing technique of system 270, shown in FIG. 5B, and therefore does not require 16 current sensing resistors.
In summary, today's implementations for LED backlighting of LCD panels with local dimming capability suffer from numerous fundamental limitations in cost, performance, features, and safety.
Highly integrated LED driver solutions require expensive large area dice packaged in expensive high pin count packages, and concentrate heat into a single package. This limits the driver to lower currents, due to power dissipation resulting from the linear operation of the current sinks, and lower voltages, due to power dissipation resulting from LED forward-voltage mismatch, a problem that is exacerbated for greater numbers of series-connected LEDs.
Multi-chip solutions combining an LED controller with discrete power MOSFETs require high BOM counts and even higher-pin-count packaging. Having nearly triple the pin count of fully integrated LED drivers, a sixteen channel solution can require 33 to 49 components and a 72 pin package as large as 14 mm×14 mm. Moreover, discrete MOSFETs offer no thermal sensing or protection against overheating.
What is needed is a cost-effective and reliable backlight system for TV's with local dimming. This requires a new semiconductor chip set that eliminates discrete MOSFETs, provides low overall package cost, minimizes the concentration of heat within any component, facilitates over-temperature detection and thermal protection, protects low-voltage components from high voltages and against shorted LEDs, flexibly scales to accommodate different size displays, and maintains precise control of LED current and brightness.